Synergistic impacts of d-delta-Tocotrienol and geranylgeraniol in human DU145 prostate carcinoma cells

The mevalonate pathway, also known as the cholesterol biosynthesis pathway, produces essential intermediates for the post-translational prenylation and dolichylation of growth-associated proteins including Ras, nuclear lamins, and growth factor receptors. Overexpression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme of this pathway, in malignant cells supports tumor growth and survival by ensuring optimal pools of nonsterol metabolites and prenylated proteins.

The naturally occurring and mevalonate-derived isoprenoids have shown reductase suppressive and anti-tumor activities both in vitro and in vivo. d-δ-Tocotrienol, a vitamin E molecule with a farnesyl side chain, and geranylgeraniol, a diterpene, accelerate ubiquitination and proteasome-mediated degradation of HMG-CoA reductase via complementary and collaborative mechanisms. We hypothesized that d-δ-Tocotrienol and geranylgeraniol synergistically inhibit growth of human DU145 prostate carcinoma cells and exert a combined suppressive effect on tumor cell cycle progression, HMG-CoA reductase expression, and Ras prenylation and membrane attachment. d-δ-Tocotrienol (IC50 = 15 μmol/L) and geranylgeraniol (IC50 = 60 μmol/L) individually induced concentration-dependent growth suppression of DU145 cells as measured by CellTiter 96® AQueous One Solution. Isobologram and Combination Index (CI) values generated by CompuSyn software demonstrated synergistic growth-inhibitory impact of the two agents in combination. Western-blot experiments showed that blends of d-δ-Tocotrienol (7.5 μmol/L) and geranylgeraniol (30 μmol/L), each at half of their respective IC50 concentrations, cumulatively suppressed level of membrane HMG-CoA reductase and putatively as a consequence of reduced prenylation, that of membrane Ras protein. Mixture of these two isoprenoids also mediated cell cycle arrest at G1 phase as indicated by flow cytometry and cell cycle distribution analysis.

Synergistic isoprenoid-based manipulation of the mevalonate pathway and tumor cell growth may offer a potential therapeutic or preventive approach to prostate cancer. This is also clinically significant since blends of isoprenoids may provide a stronger and safer long-term application.